Method of condensing blow-off steam in sulphate pulping



June 20, 1967 K. o. AXELSSON I 3,325,970

METHOD OF CONDENSING BLOW-OFF STEAM IN SULPHATE PULPING Filed Nov. 26,19,63

8 ::f 4 7/73 H I ii 15 19 SH 20 L25 Fig.2 Y 21 22 25 I I ii 3 f zoi 2VENTOR- KA RL 01.1.5 AXELSSON United States Patent METHQD 0F CONDENSINGBLOW-OFF STEAM IN SULPHATE PULPING Kari Olle Axeisson, Viggbyholm,Sweden, assignor to Aktiebolagct Rosenblads Patenter, Stockholm, Sweden,a corporation of Sweden Filed Nov. 26, 19634, Ser. No. 325,876 Claimspriority, application Sweden, Dec. 3, 1962,

12,976/ 62 1 Claim. (Cl. 55-48) The present invention relates to thecondensation of steam, herein referred to as blow-off steam, which isreleased at the blowing of a sulphate pulping digester. This blow-offsteam contains noncondensable gases such as hydrogen sulphide,mercaptans, etc. which are absorbed by the blow-oft" steam condensatebut cannot be allowed to accompany the condensate through the exhaust tothe surroundings without substantial disadvantages.

The main object of the invention is to purify, refine or clean theblow-off steam condensate by evaporating these gases out of thecondensate in the particular case where the blow-off steam is condensedduring the blowing in a known and much used type of blow-off steamcondenser system. In this system the hot condensate is fed to the upperpart of an accumulator in which it is accumulated with such circulationthat the hot condensate is collected in the upper part of theaccumulator while the cooled condensate is collected in the lower partof the accumulator. From the lower part the cooled condensate iscirculated as a cooling medium for the condensation of the steam wherebythe cooled condensate is reheated and returned to the upper part of theaccumulator.

The invention is characterized mainly in that the accumulated blow-offsteam condensate is conducted in a hot condition from the upper part ofthe accumlator without material cooling to an evaporator wherenoncondensable gases contained in the condensate are evaporated in aknown manner by means of a suitable evaporating medium, whereupon thecleaned or purified blow-off steam condensate is conducted out of thecombined blow-01f condenser-evaporator system.

The invention will now be described more fully below with reference tothe accompanying drawing in which:

FIG. 1 illustrates a condenser and evaporator set-up for practising theinvention and FIG. 2 illustrates a modified part of the arrangementaccording to FIG.1.

Each figure takes the form of a flow diagram intended only to set out anexemplary embodiment without limiting the invention. Identical elementsin the figures are indicated bv the same reference numerals.

In FIG. 1 numeral 1 indicates a condenser usually of the radiating typein which blow-oil steam, which is fed through conduit 2, is condensedand from which the condensate thus formed is conducted through conduit 3to the upper part of an accumulator 4 by way of a separator section 5.In this separator section of the accumulator the condensate is separatedfrom any residual steam and gases not absorbed by the condensate. Thisresidual steam and gas is led off through conduit 6 and possibly throughan aftercondenser which is often used but not shown in the figure. Thecondensate is collected in the accumulator to a maximum level 7determined by outlet 8 which extends from the bottom part of theaccumulator to this level where it is connected to the atmosphere byconnection 9. The condensate collected in the accumlator is circulatedfrom the upper to the lower part of the accumulator through an externalpath consisting of conduit 10, pump 11, conduit 12, control valve 13,indirect cooler 14 and conduit 15. Thereby the condensate is stratifiedso that hot condensate is collected in the upper part of the accumulatorand cooled condensate is collected in the 3,325,979 Patented June 20,1967 lower part. At each blow-off the required amount of condensate ispumped by pump 17 from the lower part of the accumulator through conduit16 and control valve 18 to act as a cooling medium for condenser 1.There the condensate is reheated in direct contact with the blow-offsteam and is led back through conduit 3 to the accumulator together withthe newly formed condensate from the blow-oft" steam. The cooling mediumfor coller 14 is supplied through conduit 19 and discharged throughconduit 20. These devices represent the essential parts of the knownblow-off condensed system to which the invention relates. Those skilledin the art know the customary details, such as the previously mentionedaftercondenser and also automatic control means, which have been omittedfrom the drawing in order to avoid unnecessarily complicating thedescription with details not essential to the invention.

The devices, which are directly required for practicing the inventionconsist of a branch conduit 21 with control valve 22 leading fromconduit 12 through which hot condensate is fed to evaporator 23 whichincludes inlet 24 for the gaseous evaporating medium, outlet 25 forclean or purified condensate and outlet 26 for the exhaust gasesconisting of noncondensable gases from the condensate mixed with theevaporating medium.

These devices are utilized according to the invention so that the amountof hot condensate circulated by pump 11 through cooler 14 isconsiderably greater than the amount of newly formed condensate.

All of the condensate which shall be removed from the system via theevaporator and outlet 25 is taken out of the amount of hot condensatecirculated by pump 11. It is essential for the invention that thecondensate arrives at the evaporator in a hot condition. For this reasonthe conduit 21 should be as short as possible and/or well insulated. Theessential advantage is that evaporating can be carried out at hightemperature and thus with high effect. Also the evaporating medium whichfor example can be air taken from the atmosphere, is supplied at lowtemperature by means of a blowing fan. This results in the lowestpossible operating costs and is especially suitable of thenoncondensable gases are to be recovered out of the exhaust gases fromthe evaporator for use in pulp production. These noncondensable gasescan be recovered by absorbing them in some process fluid, for examplewhite liquor.

In the modification according to FIG. 2 all the condensate passingthrough pump 11 is pumped to the evaporator and is returned toaccumulator 4 by pump 27 through conduit 25 with control valve 28 viacooler 14 and conduit 15. Control valve 28 is governed by the liquidlevel in the lower part of the evaporator. The advantage of using thispath for conducting the condensate is that the accumulator will containcleaned, cooled condensate which has a greater ability to absorb thenoncondensable gases out of the blow-off steam when the condensate andthe steam are fed to condenser 1. In this case all the condensate whichis to be removed from the system can be allowed to leave through outlet8 (FIG. 1) but naturally a separate outlet can also be arranged inanother place of the circulation path where the condensate is alsoclean, possibly in front of cooler 14.

A secondary, but not unimportant advantage of the invention, is that thesame evaporator utilized for the blowolf steam condensate can at thesame time be utilized also for other condensates resulting from sulphatepulping. Some of these other condensates, especially the digestercondensate, could be suitably cleaned through evaporating in which theheat supplied by the blow-01f steam contributes to the purifying ofthese condensates.

What I claim is:

In the method of condensing blow-off steam relieved during the blow-downof a sulphate pulp digester, in which said blow-off steam, containingnon-condensable gases, is condensed in a direct-contact condenser and isthereafter cooled, the hot liquid discharged from the condenser, whichconsists of a mixture of the cooling liquid supplied to the condenserand the blow-off steam condensate produced therein, containingnon-condensable gases absorbed from the blow-01f steam by the coolingliquid in the course of the condensation, said hot liquid after coolingbeing recirculated to the condenser as cooling liquid therefor, theimprovement which comprises subjecting said hot discharge liquid, beforethe cooling thereof, to a process by which a substantial portion of thenoncondensable gas contained in said liquid is removed therefrom, and,after cooling said thus treated discharge liquid, supplying it in itssubstantially degassed form to the directcontact condenser as thecooling medium therefor, the thus supplied cooling liquid absorbing asubstantial part of the non-condensable gases in the blow-01f steamwhich it encounters in the direct-contact condenser.

References Cited UNITED STATES PATENTS 2,418,167 4/1947 Du Bois 23-1782,598,116 5/1952 Du Bois 5573 2,626,005 1/1953 Sebald 55-39 3,087,7904/1963 Wheelock et al. 23-178 3,210,912 10/1965 Peake et al. 55-39 20 C.N. HART, Assistant Examiner.

